The present invention generally relates to delivery and storage systems for cryogenic fluids and, more specifically, to a cryogenic liquid delivery system that allows for rapid dispensing of cryogenic liquid directly to a use point.
As is known, cryogenic liquids are typically stored in thermally insulated tanks which consist of an inner storage vessel mounted within, and thermally isolated from, an outer shell. Cryogenic liquid is usually dispensed from a bulk supply tank to smaller storage cylinders for use in various applications. Typically, the bulk supply tank is stationary and the storage cylinders are transported to the bulk supply, refilled and transported back to the use site. During the refilling operation, flow of liquid cryogen into the storage cylinder must be terminated at the appropriate time to prevent overfilling, which may result in venting and waste of croygen. Typically, the storage cylinders are weighed as they are being filled and flow of liquid cryogen is terminated when the appropriate cylinder weight is attained.
The structural reinforcements required to ensure durability of transportable storage cylinders provide additional heat conduction paths and increase the heat inleak to the stored cryogen. In addition, transporting the tanks can be costly. There have thus been recent efforts to utilize stationary storage cylinders, which provide more insulation against heat inleak. These stationary cylinders are refilled from a transportable bulk supply tank, which may be mounted on a truck, trailer or other type of vehicle.
A variety of mobile delivery systems currently exist for providing cryogenic liquids to storage cylinders at the use point. One type of delivery system, an example of which is the HLD series manufactured by MVE, Inc. of New Prague, Minn., includes a mobile high pressure storage tank combined with a pressure-building system. Upon arrival at the use point, the pressure in the storage tank must be increased so that the fluid therein will flow to a receiving tank. This "transfer pressure" is achieved by feeding liquid from the storage tank to a pressure-building heat exchanger where it is vaporized. The vapor thus produced is routed back to the gas space above the cryogenic liquid in the storage tank so that the storage tank pressure is increased. When the storage tank has reached transfer pressure, the storage tank delivery valve and the receiving tank fill valve are opened, and the receiving tank is filled.
Upon completion of the fill, the delivery system is disconnected from the receiving tank. The delivery system storage tank must then be vented to atmosphere prior to movement to prevent the condensation of the added warmer vapor into the liquid cryogen. This condensation is caused by the motion of the liquid cryogen in the tank during transport and is undesirable as it raises the temperature of the liquid cryogen. Venting may also be necessary to reduce the tank pressure to transport levels required by Department of Transportation regulations. Venting is undesirable, however, as it takes additional time, decreases the amount of product available for distribution and is potentially hazardous. Furthermore, the venting causes a loss of cryogenic vapor which results in increased waste.
Another type of cryogenic delivery system, an example of which is the HL series available from MVE, Inc. of New Prague, Minn., utilizes a mobile storage tank combined with a pump. When cryogenic liquid flows through the pump or meter of such a system that has not been cooled down to the temperature of the cryogen, the cryogenic liquid will boil due to the heat of the pump or meter and a two-phase flow, that is, a flow that is a mixture of liquid and gas, will result. This makes pumping extremely inefficient and accurate metering for small amounts of cryogen very difficult. As such, the pump, and meter when only a small amount of cryogen is to be distributed, of such systems must be cooled down to the temperature of the cryogen before the dispensing can begin. The pumps and meters for existing systems may require as long as 30 minutes to cool down. These long cool down times result in decreased efficiency in terms of utilization of human and equipment resource time. A system that utilizes a pre-cooled pump and meter submersed in a cryogen-filled sump has been disclosed as the subject of a commonly assigned U.S. patent application (Ser. No. 08/624,268). Such an arrangement does not exist for a mobile system designed for use point distribution.
Accordingly, an object of the invention is to provide a mobile liquid cryogen delivery system for use point distribution that minimizes the pump and meter cool down period required prior to dispensing.
Another object of the invention is to provide a mobile cryogenic delivery system that minimizes the necessity of venting.
Still another object of the invention is to provide a cryogenic delivery system that features liquid cylinders with higher insulating capability, lighter weight and lower cost.